Digsby

Digsby is a ‘combo application’ that support Instant Messaging, Email and Social Networks like Twitter.
Instant Messaging Platforms supported are AIM, MSN, Yahoo, ICQ, Google Talk, Jabber, and Facebook Chat. For email it supports Hotmail, Gmail, Yahoo Mail, AOL/AIM Mail, IMAP, and POP3 accounts. Social Networking support includes Facebook, Twitter, MySpace and LinkedIn accounts.

twibble

twibble consists of 3 different products.
twibble for the desktop is a Twitter client built on top of the new Adobe AIR platform. It provides easy Twitter access in a user friendly way.

Blogo

Blogo is a weblog editor and Twitter client for Mac OS X designed for speed and ease of use. It supports Ping.fm.

DestroyTwitter

DestroyTwitter is a Twitter client built on the Adobe AIR platform. DestroyTwitter features complete direct messaging functionality. Messages and tweets can be replied to with the original visible for quick and easy reference. A search function is also available to track anything that’s being talked about.

TTYtter

TTYtter is a fully functional command-line Twitter client written using Perl.

Tweet-SQL

Tweet-SQL is a Twitter Client for Microsoft SQL Server 2005 and above. Over 30 procedures allow you to leverage the Twitter API with standard TSQL.

Twitterified

Twitterified, offers extended services on top of Twitter.
Some features:

Twitulater

Twitulater is an Adobe AIR microblogging client that currently supports Twitter and Koornk. Its aim is to make life easier for power users who are constantly bombarded by messages from the many people they follow, through automatically splitting the tweet stream into different categories based on content rather than whom the messages are from.

Twittereeze

Twittereeze is a SIMBL (Smart InputManager Bundle Loader) extension for Twitterrific , a Mac OS X client for Twitter. Twittereeze aims to improve Twitterrific’s user experience in small ways.
You need Twitterrific and SIMBL installed on your MAC OS X system for Twittereeze to function.
With Twittereeze enabled, any status that Twitterrific sends to Twitter as a [...]

TwitterPost

TwitterPost is a simple, stand-alone interface to Twitter. TwitterPost allows you to post to and read from your Twitter feed – plus a whole lot more. TwitterPost is available on Mac OS X.
It supports Unicode, which means that it has support for languages beyond just English!
Features include:

Browse your main feed as well as the Public [...]